1. Field of the Invention
The present invention relates in general to wellbore measurement tools, and in particular to logging tools for use in interrogating wellbores and surrounding formations.
2. Description of the Prior Art
As the oil and gas industry increasingly relies upon measurement-while-drilling logging operations as a full substitute for wireline logging operations, the accuracy and reliability of the measurement-while-drilling logging instruments become vital in ensuring the future of measurement-while drilling operations as a viable alternative. At present, the service-providers have a difficult time in explaining irregularities in the log data which may be due to factors other than miscalibration or malfunctioning of the logging instrument, but which can nonetheless not be easily explained. The service-recipient only sees that a block of data is somehow impaired or otherwise suboptimal, and typically wants to assess responsibility to a particular contractor or subcontractor. Generally, the service-provider is hard pressed to dispositively address these issues, and thus cannot typically repute allegations concerning log quality. Several particular sources of error are addressed by the present invention, including calibration, thermal drift, caliber operations, and mutual coupling.
In prior art systems, logging instruments are typically calibrated at the well site (or in the laboratory) during an air-hang calibration operation, during which the transmitter and receiver antennas of the logging tool are utilized to transmit and receive electromagnetic signals which propagate through the atmosphere around the tool. These air-hang calibration operations provide no data whatsoever about the operation of the tool once it is run into the wellbore and operated in the wellbore environment. Calibration values obtained during the air-hang may not apply for the wellbore environment, or the measurement-while-drilling logging tool may go out of calibration once it is run into the wellbore.
Prior art measurement-while-drilling logging instruments typically include a considerable number of analog electrical and electronic components in both the transmitting and receiving circuits, which tend to introduce an error component when subjected to changes in temperature. This type of error component is typically identified as a "thermal drift" error component. In prior art devices, this thermal drift error component introduces substantial inaccuracies in measurements, which can reduce the overall accuracy of the logging tool.
Many prior art measurement-while-drilling logging tools claim to be able to provide some indication of the size and shape of the borehole, during operations which are generally characterized as "calipering" operations. Such calipering operations depend upon the ability to detect slight changes in the amplitude attenuation or phase shift in the logging measurements which is attributable to changes in the borehole size. A variety of factors are taken into account during calipering operations, including the diameter of the logging tool, the resistivity of the drilling mud, the diameter of invasion of the drilling mud into the formation, the resistivity of the formation and drilling mud in the invaded zone, and the resistivity of the formation for uninvaded portions of the formation. Calibration errors and thermal drift error components, along with the other inaccuracies inherent in utilizing such a large number of variables typically dwarf the changes in resistivity of the borehole, and render prior art borehole calipering operations techniques essentially meaningless.
Another problem typically encountered during logging operations is undesirable magnetic field mutual coupling which may occur between two or more receiving antennas. Viewed broadly, the magnetic mutual coupling between receivers can be considered a loss of information attributable to the magnetic interaction of the receivers, and which can be considered to be an error component. More particularly, mutual coupling arises when a propagating electromagnetic field generates a current in a particular receiver, and the current which is generated in a particular receiver itself generates a propagating electromagnetic field which is combined with the interrogating electromagnetic field to influence the amount of current generated in one or more adjacent receiving antennas.
In summary, some of the principal technical problems associated with measurement-while-drilling logging tools include: (1) the inability to obtain a meaningful and accurate calibration, (2) the difficulty of obtaining the calibration, (3) the inability to determine when a tool goes out of calibration during logging operations, (4) the considerable impact on accuracy of thermal drift error components, (5) the inability to obtain accurate borehole caliber data utilizing a logging tool, principally due to the combined effect of error components associated with the variables utilized to derive borehole caliper data, and (6) the effects of undesired magnetic field mutual coupling between receiving antenna in a logging apparatus.
In the long run, in order for measurement-while-drilling logging tools to be a full substitute for wireline tools, the industry must resolve these and other problems which diminish the accuracy of the logs derived by measurement-while-drilling logging tools.